Is a vapor barrier needed?

soozAugust 12, 2012

We have a little cottage-type single family house built in the late 1940s that we rent out to a student and to a disabled person. The house is wood frame and stucco and it's on a foundation and has a crawl space.

The disabled person has kept his room enclosed without light or ventilation (there are two nicely sized, new as of 2005, windows in the room) and he keeps his little space heater on a lot.

His electrical outlet was loose so we went over there to see about a fix, and when we moved his bookcase, there was a lot of mildew stains on the wall behind the bookcase.

We're going to remove the wall and replace it, texture it, prime and paint it.

We're going to check the studs and area behind the wall to see if possibly there is any moisture coming up from the ground (remember, the house is on a foundation) and if the studs etc are dry, but at this point, I tend to think it's because the tenant lives in an environment that would support orchids or swamp flowers. I have no idea if there is insulation behind the wall or not.

Once everything is checked out and assuming it's fine (i.e. no water wicking up from the ground), would it be a good idea to install a vapor barrier (I'm not sure why, it's just something I saw on HGTV) ? If there is no insulation, do we put in the insulation and then put in the vapor barrier (if we need one) and then the wallboard/drywall/sheetrock? Would the vapor barrier only be if there was water wicking up from the ground?

I'm a bit inexperienced with this, and am at the beginning stages of my research and thought I'd stop in here first and ask some questions.

Thanks for any help or direction or insights or comments!



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Where do you live and what is the climate like?

    Bookmark   August 13, 2012 at 1:02PM
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The house is in the San Diego, CA area. Climate is mild.


    Bookmark   August 13, 2012 at 2:12PM
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Placement of a vapor barrier is highly dependent on climate. In cold climates it is placed on the inside of the insulation because Winter has moist, warm air to the interior. That moisture must be kept from the cold wall to the outside of permeable insulation. In hot, humid climates the vapor barrier, if there is one, is placed on the exterior because you do not want moisture to reach the interior, cool surface causing condensation.

Another way to look at it is that in cold climates, you want the structure to dry to the outside. I in hot, humid climates the structure must dry to the inside.

In San Diego, I have no idea what is needed. It sounds like you may, in this case, need to build your structure to suit the tenant rather then the climate.

    Bookmark   August 14, 2012 at 9:56AM
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